I thought this was going to be a £12,000 bill non-inflation adjusted, in which case I was ready to side with the PM!

I wonder what % of his clothes were actually bespoke.

My uninformed guess is all his clothes (suits, shirts, coats) were custom made, since that was the order of the day for his kind. He was an aristocrat, army, politics and writing was just adding something meaningful to his life.
Also, even people like him at that time didn't have huge wardrobes, few good peaces at any given time, and of course a strict code what to wear in the morning/evening/countryside...